The present invention relates generally to antennas for handheld communication devices, and more particularly to multiple input, multiple output antennas.
Different types of wireless mobile communication devices, such as personal digital assistants, cellular telephones, and wireless two-way email communication equipment are available. Many of these devices are intended to be easily carried on the person of a user, often fitting in a shirt or coat pocket.
As the use of wireless communication equipment continues to grow dramatically, a need exists to provide increased system capacity. One technique for improving the capacity is to provide uncorrelated propagation paths using Multiple Input, Multiple Output (MIMO) systems. MIMO employs a number of separate independent signal paths, for example by means of several transmitting and receiving antennas.
This typically requires multiple antennas which results in duplication of certain parts within the wireless mobile communication device, and results in an unfavorable trade-off between device size and performance. The trade-off is that smaller devices suffer performance problems, including shortened battery life and potentially more dropped calls, whereas devices with better performance require larger housings. The primary factor of this trade-off is mutual coupling between the antennas, which can result in wasted power when transmitting and a lower received power from incoming signals.
Effective MIMO performance requires relatively low correlation between each signal received by the multiple antennas. This is typically accomplished in large devices using one or more of: spatial diversity (distance between antennas), pattern diversity (difference between antenna aiming directions), and polarization diversity.
Unfortunately, when multiple antennas are used within a mobile handheld communication device, the signals received by those antennas are undesirably correlated, due to the tight confines typical of the compact devices that are favored by consumers. This noticeably disrupts MIMO performance. The trade-off is then to either enlarge the device, which consumers will likely shun, or else tolerate reduced performance.
Therefore, is it desirable to develop an MIMO antenna arrangement which is capable has a compact size to fit within a device housing small enough to be desired by consumers and which has improved performance.